Children raised in a house with two or more dogs or cats during the first year of life may be less likely to develop allergic diseases as compared with children raised without pets, according to a study in the August 28, 2003 issue of the Journal of the American Medical Association.

Exposure and immune response

"The striking finding here is that high pet exposure early in life appears to protect against not only pet allergy but also other types of common allergies, such as allergy to dust mites, ragweed, and grass," says Marshall Plaut, MD, chief of the allergic mechanisms section at the National Institute of Allergy and Infectious Diseases, which supported the study along with the National Institute of Environmental Health Sciences. "Other studies have suggested a protective effect of pet exposure on allergy and asthma symptoms, but generally have looked only at whether pet exposure reduced pet allergy. This new finding changes the way scientists think about pet exposure; scientists must now figure out how pet exposure causes a general shift of the immune system away from an allergic response."

In their paper, lead author Dennis R. Ownby, MD, of the Medical College of Georgia, and colleagues suggest that bacteria carried by pets may be responsible for suppressing the immune system's allergic response. These bacteria release molecules called endotoxins, and endotoxins are believed to shift the developing immune system away from responding to allergens through a class of lymphocytes called Th-2 cells, which are associated with allergic reactions. Instead, endotoxins may stimulate the immune system to activate Th-1 cells, which may block allergic reactions.

The researchers followed 474 children from birth to six or seven years of age. When the children were one year old, the researchers contacted parents by telephone to find out how many pets were in the home. When the children were two years old, researchers measured the level of dust mite allergen in their bedrooms. When the children were six or seven, the researchers tested them for allergic antibodies to common allergens by two approaches -- a skin prick test and a blood measurement.

After adjusting for factors such as dust mite allergen levels, parental smoking, and current dog or cat ownership, the researchers found that children exposed to two or more dogs or cats during the first year of life were on average 66 to 77 percent less likely to have any allergic antibodies to common allergens, as compared with children exposed to only one or no pets during their first year. "Our findings suggest an area of research with many possibilities, one that could potentially bear fruit over the next decade or so," says Dr Ownby. "If we could find out exactly what it is about pets or the bacteria they carry that prevents the allergic response, scientists might be able to develop a new allergy therapy based on that knowledge."